The present invention relates to the field of interconnecting computers and their peripherals via a serial bus. More specifically embodiments of the present invention relate to the field of isolating a Universal Serial Bus (USB) host device from a plurality of USB peripheral devices.
In a typical computer system, a computer with a microprocessor may be connected to a plurality of peripheral devices (e.g., printer, mouse, modem, scanner, test equipment etc. . . . ) by one or several electrical buses and through several different ports. Different peripheral devices have interfaces with different form factors and different electrical characteristics, and these communications ports are used for variety of different peripheral devices. Each peripheral device may be connected to the computer with a dedicated port and each one of these dedicated ports may have different electrical characteristics. These different electrical characteristics (i.e., serial, parallel or other mode of communication) may require different connectors and different cables, which run from the peripheral to the port and from the port to the microprocessor. The necessity for having a different cable for each of the peripherals connected to a host computer resulted in an assortment of cable connectors connected to a host computer.
The requirements for different connectors for different peripherals led to the development of standardized cables and protocol capable of connecting different peripherals to a host computer. One emerging design, responding to the need for a reduced number of cable connectors, is the Universal Serial Bus (USB). A universal serial bus is a standardized bus used to connect different peripherals with different interface requirements to a host computer, thus replacing the method of using different connectors for different port capabilities. A USB replaces the multiple cable and connector types with a single standardized connection system.
The USB also permits the connection and disconnection of USB compatible peripheral devices while the computer is turned on. Prior to implementation of USB, connecting a peripheral to a host required turning off and rebooting the host computer. USB eliminates the typical turning off and rebooting procedures and a peripheral can be connected without the need for turning off and rebooting of the host device.
A host computer capable of communication with its peripheral via the USB is generally known as a USB host and the peripherals supporting USB technology are known as USB peripheral devices. Most USB host computers support a plurality of USB peripherals transacting with a USB host computer through a number of different electrical signals. A typical USB peripheral device may be a keyboard, a mouse, a sound system or even an oscilloscope or test equipment. Virtually any peripheral can be developed having a USB port interface.
Generally, peripheral devices, connected to a host computer, are electrically isolated from the host device for variety of reasons. In many instances, ground currents will flow between a host computer and the peripheral connected to it and induce significant noise in certain types of devices. For example, an oscilloscope will typically measure down into the microvolt range and ground loop hum will severely affect the ability of the instrument to operate. High quality audio devices and medical instruments are also approaching this realm of electrical sensitivity. Therefore, electrical isolation of the peripheral from the host helps reduce or eliminate ground loop hum or other types of signal noise on the bus.
In devices operating with high voltage, safety is an important factor. For a device such as monitor, oscilloscope, test equipment, or LCD backlight the risk of high voltage spikes and static discharge causing destruction of the USB host are ever present. Furthermore, in cases where a plurality of peripherals are coupled to a host computer, malfunctioning of one device may affect the operation of other devices. Often, a difference in electrical requirements or a difference in functions performed by one peripheral device may have an adverse affect on other devices unless they are isolated. To reduce or even eliminate the hazards described above, attempts have been made to isolate peripheral devices from host devices.
Therefore, the USB works well for (1) peripherals that are bus powered (2) do not contain sensitive elements, and USB devices for which (3) their use does not have the ability to apply voltage over the potential of the bus voltage. However, for devices that violate these generalities, isolating the USB host from the USB device may provide a design improvement.
Conventional Art FIG. 1 depicts a conventional coupling of a peripheral device 150 and a host computer 110 which are electrically isolated yet capable of communication.
In system 100, peripheral device 150 is connected to host device 110 via a DC connection of four signal lines, 111-114. Signal lines 112 and 114 are data transmission lines. Ground signal 111 is connected between host device 110 and peripheral 150, and power signal 113 draws power from host device 110. Isolation device 130 isolates host device 110 from the rest of the peripheral system 140. According to the conventional method used by the industry, isolation of host device 110 from the rest of the peripheral system 140 may be achieved by isolation of the plurality of I/O signals 180/160 individually. Each I/O signal from peripheral device 140 is optically isolated by isolators 130.
Isolating a peripheral from a host computer helps achieve the goal of securing safety of the computer operator as well as preventing ground currents flow between host and peripherals. Flow of such current induces significant noise in certain types of sensitive devices, in devices that measure in micorvolt range a ground loop severely affects the ability of the instrument to operate.
The problem with system 100 is that it requires many of the I/O signal lines 180 and 160 to be isolated individually. In many devices, the isolation circuitry is the most expensive part of the design. Therefore, what is desired is an interface that provides isolation of the USB while limiting or reducing the number of signal lines that need to be isolated.
Therefore, a need exists to design a device allowing different USB peripherals to connect to a host device via a single connector which provides electrical isolation in a safe, economical and electronically efficient manner.
Accordingly, an embodiment of the present invention provides an apparatus which enables a USB host computer to economically isolate a plurality of coupled USB peripheral devices. The USB peripheral devices are equipped with individual optical isolation barriers with a reduced number of isolation elements. Another embodiment of the present invention provides protection against electrical noise inherent to a device supporting a plurality of peripherals with a wide range of voltage requirements and electrical characteristics. As the result of reduced number of isolating elements substantial economic gains can be realized.
A peripheral device having bus isolation from a host computer is disclosed. The peripheral device has a microcontroller, which interfaces with a plurality of input output (I/O) signals from a plurality of logic devices in the peripheral device. The microcontroller translates the plurality of I/O signals to signals compatible with a serial bus. The peripheral device further includes a first unidirectional to bi-directional converter which receives the signals from the microcontroller and directs the signals in accordance with an instantaneous directional bus signal from the microcontroller. An optical isolation barrier comprises a number of optical isolating devices, which isolate these bus compatible signals from another unidirectional to bi-directional converter. The second unidirectional to bi-directional converter has a separate ground potential than the first unidirectional to bi-directional converter and is coupled to a USB interface. The second converter supplies isolated bus signals.
The present invention advantageously locates the isolation barrier on the USB signals rather than the peripheral I/Os, thereby needing only to isolate a limited number of signals. This may lead to a more economical result. The isolation can be performed using optical elements. The separation of ground potentials between the two converters and isolation of the peripheral device from the host computer with fewer optical isolation circuits substantially reduces noise, ground loop and possible electrical hazards along with possible substantial reduction in cost.
More specifically, a peripheral device with bus isolation is disclosed. The peripheral device includes a microcontroller for receiving a plurality of input/output (I/O) signals from peripheral device logic. The microcontroller translates the plurality of I/O signals to signals compatible with a serial bus and for transmission over the serial bus. The number of signals transmitted over the serial bus are fewer than the plurality of I/O signals received by the microcontroller. The peripheral device further includes a first unidirectional to bi-directional converter, which is electrically coupled to the microcontroller. In one embodiment of the present invention, an optical isolation barrier is coupled to the first unidirectional to bi-directional converter via unidirectional signal lines. A second unidirectional to bi-directional converter is then coupled to the optical isolation barrier via isolated unidirectional signal lines. The second converter generates isolated bus signals and is then coupled to a serial bus interface for interfacing with a host device using said plurality of I/O bus signals.
The microcontroller generates a bus direction signal to indicate if the microcontroller is sending or receiving data. This signal is supplied to both converters so that proper bi-directional to unidirectional and, vice-versa, conversion can be accomplished. In one implementation, this signal originates from a dedicated pin on the microcontroller. A USB host may also have the ability to indicate the bus direction across the isolated barrier.
These and other objects and advantages of the present invention will no doubt becomes obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures.